This invention relates to a method of checking the functioning of a control system installed in an automotive vehicle, and more particularly to a method of checking the functioning of a driving wheel slip control system.
As recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle is started to run or when it is accelerated, if the driving force of the driving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [=the coefficient of friction between the tire and the road surface x load of the vehicle weight on the driving wheel (wheel load)].
Driving wheel slip control systems for controlling slips of driving wheels have already been proposed by the present assignee, e.g. in Japanese Provisional Patent Publications (Kokai) Nos. 2-157439 and 2-157440. According to these systems, driving wheel slip control is carried out by the following steps:
1) detecting the rotational speeds of right and left driving wheels, the rotational speeds of right and left trailing wheels, and the steering angle of a steering handle,
2) calculating a parameter (hereinafter referred to as "the slip value") indicative of the degree of a slip of the driving wheels based on the detected rotational speeds of the wheels and steering angle, and
3) increasing the number of cylinders of an internal combustion installed in the vehicle to be subjected to fuel cut to thereby decrease the output torque of the engine as the slip value is larger (the degree of the slip is larger).
The detection of the steering angle carried out at the above step 1) is for controlling the yawing motion of the vehicle based on the detected values of the steering angle and rotational speeds of the right and left trailing wheels. This control of the yawing motion is carried out by decreasing the output torque of the engine, e.g., for the purpose of reducing the tendency of the front-wheel-drive vehicle during yawing thereof toward understeering. The yawing motion control is a kind of driving wheel slip control in a broad sense.
On the other hand, a system for checking functioning or operation of such driving wheel slip control systems has already been proposed by Japanese Provisional Utility Model Publication (Kokai) No. 63-84544, in which all the four wheels of a vehicle are placed on respective drum rollers capable of rotation independently of one another (these drum rollers, the equivalent inertial weights of which are small, play the role as a frozen road or the like), and it is determined that the driving wheel slip control system of the vehicle is not normally functioning if the rotational speeds of the right and left driving wheels exceed a predetermined upper limit value within a predetermined time period after the accelerator pedal of the vehicle is stepped on.
According to the above proposed system, the checking is so roughly carried out that it is only possible to determine whether the driving wheel slip control system has operated or not. However, it is not possible to determine whether driving wheel slip control systems like the aforementioned ones proposed by the present assignee which are capable of sensitive control are properly functioning to perform varions kinds of control as designed. More specifically, the above proposed checking system cannot determine e.g. whether the slip value is properly calculated based on the rotational speeds of the right and left driving wheels, those of the right and left trailing wheels, and the steering angle of the steering handle, and whether fuel cut and/or other control are properly carried out based on the calculated slip value.
Further, since the conventional checking system uses data based solely on the rotational speeds of the wheels, it is impossible to determine which part of the control system is faulty when it is determined to be out of order.